Pick mechanisms for transferring sheet media are known and usually comprise one or more suction cups mounted on a pick means which comprises a pivoting arm and a tube (known as a pick line) opening into the suction cup. The pivoting arm brings the suction cup into contact with sheet media, and a pump is used to suck air through the pick line creating a partial vacuum. The suction cup is then moved away with the media sheet held thereto by suction.
Taking the example of an Automated Teller Machine (ATM), in which the sheet media comprises currency notes, most ATMs include a plurality of vacuum picking mechanisms so that different denominations of currency notes can be dispensed from stacks of notes of that denomination. In the UK, typically four modules are provided, two of which may be dedicated to picking from stacks of twenty-pound notes and the other two of which may be dedicated to picking from stacks of ten-pound notes.
The strength of the vacuum required may be calculated according to the media to be picked. Returning to the example of ATMs, a pump must be powerful enough to pick up one note from the front of a stack, but not so powerful that two notes are picked up. A key factor in how securely the note is held is the difference between the pressure within the cup and the local atmospheric pressure. At present, a vacuum is created according to predetermined criteria dependent on physical attributes of the currency and denomination with which the pick mechanism is intended to be used.
In prior art pick means, the pump is controlled to perform a predetermined number of strokes, or parts of a stroke depending on the pump, to produce the pressure, subject to local atmospheric variations, which it has been previously determined is suitable for picking the media types with which it is intended that the pick means be used. This may result in pick failure where, for example there is a hole in the sheet media through which air can ingress. Alternatively, the pressure may drop as the note is transported by the pick means and the sheet media may detach from the suction cup.
In order to create a reduction in pressure relative to atmospheric pressure, the volume of a sealed system may be increased. To halve the pressure, the volume must be doubled and therefore the volume in which the partial vacuum is created is preferably as small as possible. This means that a smaller pump can be used to achieve an acceptable picking time and/or the strokes made by a pump can be fewer or shorter. The benefit in creating a vacuum quickly is both in the speed with which media is picked up and handled and also in the likely success of a picking attempt. Creating a pressure difference quickly is more likely to ‘suck’ the media into firm contact with the suction cup and create a good initial seal.